ctty_get_name(): fix buffer overrun due to incorrect use of strncpy()

[srd424]

strncpy() will zero-fill the destination up to the size passed - MAX_PATH_LEN, but we only malloc() the actual size of the string, so this overruns and causes heap corruption.

I don't fully understand your approach to string handling, but it seems far easier just to use strdup() here!

Fixes: https://github.com/emptymonkey/shelljack/issues/3